Re: [PATCH RFC v5 12/12] hw/block/nvme: add persistence for zone info

From: Klaus Jensen <its@irrelevant.dk>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <fam@euphon.net>,
	qemu-block@nongnu.org, Klaus Jensen <k.jensen@samsung.com>,
	qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
	Keith Busch <kbusch@kernel.org>
Subject: Re: [PATCH RFC v5 12/12] hw/block/nvme: add persistence for zone info
Date: Mon, 30 Nov 2020 14:18:42 +0100	[thread overview]
Message-ID: <X8TxMnoP/YOn7/hc@apples.localdomain> (raw)
In-Reply-To: <X8TspZJE+B551qno@apples.localdomain>

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On Nov 30 13:59, Klaus Jensen wrote:
> On Nov 30 12:33, Stefan Hajnoczi wrote:
> > On Fri, Nov 27, 2020 at 12:46:01AM +0100, Klaus Jensen wrote:
> > > From: Klaus Jensen <k.jensen@samsung.com>
> > > 
> > > Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
> > > ---
> > >  docs/specs/nvme.txt   |  15 +++
> > >  hw/block/nvme-ns.h    |  16 ++++
> > >  hw/block/nvme-ns.c    | 212 +++++++++++++++++++++++++++++++++++++++++-
> > >  hw/block/nvme.c       |  87 +++++++++++++++++
> > >  hw/block/trace-events |   2 +
> > >  5 files changed, 331 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/docs/specs/nvme.txt b/docs/specs/nvme.txt
> > > index 03bb4d9516b4..05d81c88ad4e 100644
> > > --- a/docs/specs/nvme.txt
> > > +++ b/docs/specs/nvme.txt
> > > @@ -20,6 +20,21 @@ The nvme device (-device nvme) emulates an NVM Express Controller.
> > >    `zns.mor`; Specifies the number of open resources available. This is a 0s
> > >       based value.
> > >  
> > > +  `zns.pstate`; This parameter specifies another blockdev to be used for
> > > +     storing zone state persistently.
> > > +
> > > +       -drive id=zns-pstate,file=zns-pstate.img,format=raw
> > > +       -device nvme-ns,zns.pstate=zns-pstate,...
> > > +
> > > +     To reset (or initialize) state, the blockdev image should be of zero size:
> > > +
> > > +       qemu-img create -f raw zns-pstate.img 0
> > > +
> > > +     The image will be intialized with a file format header and truncated to
> > > +     the required size. If the pstate given is of non-zero size, it will be
> > > +     assumed to already contain zone state information and the header will be
> > > +     checked.
> > 
> > In principle this makes sense but at first glance it looks like the code
> > is synchronous - it blocks the vCPU if zone metadata I/O is necessary.
> > That works for test/bring-up code but can't be used in production due to
> > the performance impact.
> > 
> > Is the expectation that the QEMU NVMe device emulation will only be used
> > for test/bring-up now and in the future?
> > 
> 
> Hi Stefan,
> 
> Thanks for taking a look at this.
> 
> I could see why someone would maybe use the core nvme emulation in
> production (but I'm not aware of anyone doing it), but the zoned
> emulation is *probably* not for production (and that is where the zone
> updates are needed). But someone could surprise me with a use case I
> guess.
> 
> And yes, I know this is synchronous in this version. I have not
> extensively evaluated the performance impact, but crucially the blocking
> only happens when the zone changes state (i.e. every write does NOT
> trigger a blk_pwrite just to persist the updated write pointer).
> 
> I know this can be done asynchronously (I have implemented it like so
> previously), but I wanted to get an opinion on the general stategry
> before adding that. The opposing strategy, is to use a some form of
> mmap/msync, but I, for one, pushed back against that because I'd like
> this to work on as many platforms as possible. Hence the RFC for this
> blockdev based approach.
> 
> But if you think a blockdev approach like this is a reasonable QEMU-esce
> way of doing it, then I'll proceed to do a v2 with asynchronous updates.

Let me rephrase that I will most likely wait to do the v2 until we have
conconsus and reviews on a zoned series.

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